1. Field of the Invention
The present invention pertains to telecommunications, and particularly to any cellular/mobile telecommunications using diversity (soft) handover, such as occurs in spread spectrum or code division multiple access (CDMA) technology.
2. Related Art and Other Considerations
In mobile telecommunications, a mobile station such as mobile telephone communicates over radio channels with base stations. Typically a plurality of base stations are, in turn, ultimately connected to a mobile switching center. The mobile switching center is usually connected, e.g., via a gateway, to other telecommunication networks, such as the public switched telephone network.
In a code division multiple access (CDMA) mobile telecommunications system, the information transmitted between a base station and a particular mobile station is modulated by a mathematical code (such as spreading code) to distinguish it from information for other mobile stations which are utilizing the same radio frequency. Thus, in CDMA, the individual radio links are discriminated on the basis of codes. Various aspects of CDMA are set forth in Garg, Vijay K. et al., Applications of CDMA in Wireless/Personal Communications, Prentice Hall (1997).
In addition, in CDMA mobile communications, typically the same baseband signal with suitable spreading is sent from several base stations with overlapping coverage. The mobile terminal can thus receive and use signals from several base stations simultaneously. Moreover, since the radio environment changes rapidly, a mobile station likely has radio channels to several base stations at the same moment, e.g., so that the mobile station can select the best channel and, if necessary, use signals directed to the mobile from various base stations in order to keep radio interference low and capacity high. This utilization of radio channels from multiple base stations by a mobile station in a CDMA scheme is termed "soft handover."
FIG. 1 shows a radio access network (RAN) 20 which comprises radio network controllers (RNC) 22.sub.2 and 22.sub.2 respectively connected to mobile switching centers (MSC) 24.sub.1 and 24.sub.2. Radio network controller (RNC) 22.sub.1 is connected to base stations (BS) 26.sub.1,1, 26.sub.1,2, and 26.sub.1,3 ; radio network controller (RNC) 22.sub.2 is connected to base stations (BS) 26.sub.2,1, 26.sub.2,2, and 26.sub.2,3. At the moment shown in FIG. 1, and for reasons summarized above, mobile station MS is shown in FIG. 1 as having radio communication with two base stations, particularly base stations 26.sub.1,2, and 26.sub.1,3. The lines 28.sub.1,2 and 28.sub.1,3 each represent a communication path. Specifically, line 28.sub.1,2 depicts both the radio channel from mobile station MS to base station BS 26.sub.1,2 and the land line link channel from base station BS 26.sub.1,2 to radio network controller (RNC) 22.sub.1 ; line 28.sub.1,3 depicts both the radio channel from mobile station MS to base station BS 26.sub.1,3 and the land line link channel from base station BS 26.sub.1,2 to radio network controller (RNC) 22.sub.1. In the case of both lines 28.sub.1,2 and 28.sub.1,3, the land line link is connected to a diversity handover unit (DHU) 30.sub.1 of radio network controller (RNC) 22.sub.1.
Thus, as depicted with reference to FIG. 1, the mobile connection with mobile station MS potentially utilizes several "legs", each leg being represented by the lines 28.sub.1,2 and 28.sub.1,3 in the case of mobile station MS of FIG. 1. As the overall connection between mobile station MS and any other party is viewed, the diversity handover unit (DHU) 30.sub.1 serves essentially both to combine and split the different legs utilized by a mobile station. The splitting occurs in the sense that information directed toward the mobile station is directed along the plural parallel legs to differing base stations. Information received from a base station may actually be obtained through several of the legs (e.g., from several base stations), in which sense the diversity handover unit (DHU) 30.sub.1 serves a combining function.
FIG. 1 illustrates the simple case in which the different legs of the connection, represented by lines 28.sub.1,2 and 28.sub.1,3, are for base stations BS all of which are connected to radio network controller (RNC) 22.sub.1. However, should the mobile station MS roam sufficiently to pick up signals from another base station controlled by another RNC, e.g., into or proximate a cell handled by base station BS 26.sub.2,1, for example, a more complex situation occurs as shown in FIG. 1A.
In the situation depicted in FIG. 1A, the mobile connection involving mobile station MS employs base stations belonging to differing radio network controllers (RNC). Such situation involves a different type of handover--an inter-RNC soft handover. Inter-RNC soft-handovers are made between two or several RNCs. In the particular situation shown in FIG. 1A, an inter-RNC soft handover is made between radio network controller (RNC) 22.sub.1, which is also known as the "Source" RNC, and radio network controllers (RNC) 22.sub.2, which is also known as the "Target" RNC. Radio network controller (RNC) 22.sub.2 is the Source RNC since it has current control of the mobile radio connection. The Target RNC is an RNC, other than the Source RNC, that has, or has been decided to have, base stations utilized by the mobile radio connection.
To facilitate, e.g., inter-RNC soft-handovers, the radio network controllers (RNC) 22.sub.2 and 22.sub.2 are connected by an inter-RNC transport link 32. Inter-RNC transport link 32 is utilized for the transport of control and data signals between Source RNC 22.sub.1 and Target RNC 22.sub.2, and can be either a direct link or a logical link as described, for example, in International Application Number PCT/U.S. Ser. No. 94/12419 (International Publication Number WO 95/15665).
Thus, in FIG. 1A, the mobile station MS communicates not only through the leg represented by line 28.sub.1,3, but now also by the leg represented by line 28.sub.2,1. The leg represented by line 28.sub.2,1 includes the radio link between mobile station MS and base station BS 26.sub.2,1, as well as the information pertinent to the mobile connection which is carried over inter-RNC transport link 32.
As mobile station MS continues to move, it may eventually occur that all of the base stations utilized by the mobile station are served by the Target RNC 22.sub.2, as is pictured in FIG. 11B. In such case, inter-RNC transport link 32 must carry both legs of the mobile connection, represented by lines 28.sub.2,1 and 28.sub.2,2, respectively. Carrying multiple legs of the same mobile connection undesirably demands further resources from inter-RNC transport link 32. In FIG. 1B, diversity handover unit 30.sub.1 handles all combining and splitting operations, even though no base station owned by Source RNC 22.sub.2 is employed by the mobile connection with mobile station MS.
For the situation shown in FIG. 1B, resources of inter-RNC transport link 32 can be preserved if the diversity handling operations are moved to Target RNC 22.sub.2. Utilization of inter-RNC transport link 32 would be reduced in that, for example, multiple packets destined in parallel for base stations BS 26.sub.1,2 and 26.sub.2,2 need not be carried on link 32, but rather a diversity handover unit at Target RNC 22.sub.2 could instead perform the splitting. A similar economy results in having a diversity handover unit at Target RNC 22.sub.2 combining the signals from mobile station MS as received via the base stations BS 26.sub.2,1 and 26.sub.2,2, and forwarding a resultant signal to Source RNC 22.sub.1.
The movement of diversity handling operations (diversity handling "moveover") to a Target RNC, such as Target RNC 22.sub.2 of FIG. 1B, is a complex endeavor, and can potentially result in an interruption of the established mobile connection. The prior art approach to moving diversity handling operations is shown in International Application Number PCT/U.S. Ser. No. 94/12419 (International Publication Number WO 95/15665). That approach, illustrated in FIG. 1C, involves a two step process. The first step of the process is routing the mobile connection, in a bypass mode, through a diversity handling unit (DHU) 30.sub.2 in the Target RNC 22.sub.2 upon first utilization by mobile station MS of the Target RNC (e.g., when a base station served by Target RNC is first invoked). In its bypass mode, diversity handling unit (DHU) 30.sub.2 in the Target RNC 22.sub.2 performs no combining or splitting operations. Rather, all combining and splitting operations remain the province of diversity handling unit (DHU) 30.sub.1 in Source RNC 22.sub.2.
Only later, when (and if) all base stations utilized by mobile station are owned by the Target RNC 22.sub.2, is the second step of the above-described approach implemented. In the second step, the combining and splitting functions are moved from diversity handover unit (DHU) 30.sub.1 of Source RNC 22.sub.2 to diversity handover unit (DHU) 30.sub.2 of Target RNC 22.sub.2, and diversity handover unit (DHU) 30.sub.1 is bypassed as shown in FIG. 1C.
Prior art techniques of diversity handling moveover are problematic for several reasons. For example, at the time shown in FIG. 1B prior to actual transfer of combining/splitting operations to Target RNC 22.sub.2, two transport connections (e.g., two legs of traffic for a single connection) are still wastefully used on inter-RNC transport link 32. Moreover, for each RNC that is used by the mobile radio connection, one diversity handover unit (DHU) 30 is necessary. The use of two diversity handover units (s) is a further waste of hardware.
What is needed therefore, and an object of the invention, is an efficient and economical diversity handling moveover technique.